Sieve analysis

The process of dividing a sample of aggregates into fractions of the same particle size is known as a sieve analysis. The purpose of a sieve analysis is to determine the paste requirement for a workable concrete since the amount of void needs to be filled by the same amount of cement paste in a concrete mixture. In a sieve analysis, a sample of dry aggregate of known weight is graded by shaking or vibrating through a series of sieves with progressively smaller openings, with the largest sieve at the top, for a specified time. Particle size distribution is expressed in terms of the percentage by weight passing the various sieves.

Proper gradation (or size distribution) of aggregates is one the most important factors in producing workable concrete. Proper gradation ensures that a sample of aggregate contains all standard fractions of aggregate in the required proportion, such that the sample contains minimum voids. A sample of a well-graded aggregate containing minimum voids will require minimum paste to fill up the voids in aggregates. Minimum paste means a lower amount of cement and less water, leading to an increased economy, higher strength, lower shrinkage, and greater durability.

The figure below shows particle compaction in the case of poorly graded and well-graded aggregates and the relative quantity of cement that might be used.

Figure: Grading of aggregates

Fine and coarse aggregates should be graded in dry conditions. The sample size should be representative of the whole consignment. The aggregates should be air dried and of sufficient quantity, as specified in the Table below to produce accurate result shall be used.

Table: Minimum weights of graded samples for sieve analysis